1. Field of the Invention
This invention relates generally to the field of optical imaging. More particularly, the invention relates to the field of imaging through improved beam source and lens configurations.
2. Description of the Related Art
Many optical devices involve collecting light from a source (such as a laser) and manipulating it with a lens system. The lens system may be used to project the light as a collimated beam or to focus the light to a concentrated spot. For example, optical scanning of a surface is generally done with a focused spot to provide localized interaction with a small area of the surface at any one time. Optical scanners have been developed for a variety of uses including optical data storage, bar code readers, image scanning for digitization or xerography, laser beam printers, inspection systems, densitometers, and 3-dimensional scanning (surface definition, surface characterization, robotic vision). In these scanning applications, the light spot is scanned over a surface for either xe2x80x9creadingxe2x80x9d the surface (i.e., producing a return signal that carries information about some local property of the surface) or xe2x80x9cwritingxe2x80x9d to the surface (i.e., causing a localized change in a property of the surface material). In a typical compact disk (CD) reader, light from a discrete laser diode is focused by a lens system to illuminate a spot that scans a single data track of the CD.
In many optical devices that image a source to a scanning spot, the rate at which information is read or written can be increased through the use of multiple sources and multiple spots. Zen Research, Inc. has recently introduced a CD-ROM drive that uses a diffraction grating to split the output beam of a single conventional diode laser into seven parallel beams that are focused to spots on seven adjacent data tracks of the CD. The sources for the seven spots are images of the laser created by the grating. The return signals from the spots are read by a multiple element detector to give seven parallel, simultaneous read channels. Only the center beam is used for focusing and tracking because the other beams are sent through the same objective lens and are focused and aligned with their respective tracks along with the center beam. That system is for reading only.
Summary of the Problem
There is a need in the art for an inexpensive, near diffraction-limited optical system that provides independently controllable spots (e.g., for writing), such as those wherein light for the multiple sources originates from different lasers. Current optical writing systems that meet the requirements for compactness, incorporate a single-lens system for collecting and focusing light from the multiple sources. Within these conventional single-lens systems, satisfactory (i.e., near-diffraction-limited) focusing performance has been achieved when the multiple sources are placed within a field of view limited by the lens system design. Further, while most aberrations in inexpensive lenses can be corrected using modem molding technology to reproduce aspheric or binary-diffraction lens surfaces, curvature of field is only correctable with costly lens designs involving many elements.
Summary of the Solution
The present invention provides a method for imaging light from a set of integrated optical light guides arranged over an extended field of view with a simple, inexpensive lens system. The invention contributes to high performance in the form of a reliable, robust, manufacturable, low-cost component for optical devices used for optical data storage, bar code readers, image scanning for digitization or xerography, laser beam printers, inspection systems, densitometers, and 3-dimensional scanning (surface definition, surface characterization, robotic vision).
Integrated optics can be used to direct multiple light sources through parallel light guides toward the same focusing lens, which images the light guide exit apertures onto separate spots. Re-imaging returns the light reflected from each spot to the light guide from which it originated, where it can be separately detected. With integrated optics, the number of parallel light guides acting as sources can be increased as necessary. However, the light guides must occupy different positions in the field of view of the lens. Using integrated optics fabrication methods, it is easiest to arrange them in a straight line so that the light guides can be fabricated in a single flat sheet of material and terminate with their exit apertures along one edge facing the lens. If the light guides are spaced at a constant distance from each other, every time a light guide is added to increase the number of sources, it occupies a position in the lens field of view that is further away from the lens axis. For a simple lens system, the number of light guides that can be satisfactorily imaged is limited to those within the field of view set by curvature of field effects. In addition, as sources are positioned further from the optical axis of the lens system, their light may be partially cut off by the limiting aperture of the lens system (an effect called vignetting) if the axis of the emitted cone of light is still parallel to the optical axis. Lens systems can be designed that accept light cones parallel to the optical axis without vignetting; such systems may be oversize, telecentric, or both, resulting in increased bulk and cost. A single lens requires light cones that are directed at the center of the lens, so that the entire lens is illuminated for best light efficiency and smallest focal spot size. A simple lens system may require light cones directed either toward or away from the optical axis, depending on system details; the light must be centered on the limiting aperture of the system, which may not be the first lens encountered.
The present invention extends the performance of a simple lens system with curvature of field to image a set of optical light guide sources extended over a wide field of view. The invention is applicable to systems using integrated optical light guides as the sources for imaging. It can allow such systems to use a wide field of view with simple aspheric lenses with resulting savings in complexity, size, mass, and cost.
Other features, advantages, and embodiments of the invention are set forth in part in the description that follows, and in part, will be obvious from this description, or may be learned from the practice of the invention.